1. Field of the Invention
The present invention relates generally to a solenoid control systems, and more specifically to an apparatus and method for controlling the operation of a solenoid.
2. Description of the Prior Art
Electromechanical solenoids provide a mechanical action in response to an electrical signal. Such electromechanical solenoids typically consist of an electromagnetically inductive coil wound around a movable ferromagnetic core or armature. The coil is configured to allow linear motion of the armature in response to an applied energizing signal in order to apply a mechanical force to some external mechanism or electromechanical device. A spring is typically provided to reset the armature to its original position when an energizing signal is removed. In a typical application, an electrical energizing signal is provided to the solenoid coil in response to a manual operation such as the operation of a pushbutton switch. In other applications, a logic device is used to provide an energizing signal to the solenoid in response to a predetermined condition. In many applications, a sensor is utilized to sense a condition of an external mechanism acted upon by the solenoid, and switches are then used to then deenergize the solenoid.
In some cases, the energizing signal to the solenoid is inadvertently maintained for an extended period, often due to a delay in the operation of the desired external mechanical operation. In other examples, a manual switch is held in the closed position and continues to provide an energizing signal after solenoid operates, or, there is an unexpected mechanical delay in the solenoid operation after the signal is provided. In such cases of maintained energizing signals, the solenoid can fail from overheating due to extended current flow.
One way to avoid failure of a solenoid due to such overheating would be to use a larger more robust solenoid device. However, this adds additional cost and requires more physical space than may be available.
Another way the problem of solenoid overheating has been addressed has been to employ a control circuit for the solenoid that is configured to shut off the energizing signal to the solenoid after a predetermined time. For example, a monostable multivibrator is used to supply an electrical signal to a solenoid upon receipt of a switching initiation signal. The duration of the output pulse is controlled to be sufficiently long enough to properly operate the solenoid without overheating in most instances. Such methods protect the solenoid but are not capable of overcoming a delay in the operation of the solenoid or the desired external mechanical operation because the solenoid energizing signal is cut off after a predetermined time period.
In view of the foregoing considerations, there is a need to provide a solenoid control circuit that protects a solenoid from overheating, and is capable of overcoming a delay in operation by automatically re-energizing the solenoid for one or more predetermined periods until the desired operation is effected.
The present invention will be apparent to those skilled in this art from the following detailed description of a preferred embodiment of the invention.